This invention generally relates to a multichannel image display apparatus and more particularly to an apparatus and method for equalizing drive voltage provided over multiple channels to a spatial light modulator.
Spatial Light Modulator (SLM) devices are increasingly being used in a wide range of imaging applications such as digital projection and printing. Typical spatial light modulators include devices such as Liquid Crystal Devices (LCDs) and digital micro-mirror devices (DMDs). A spatial light modulator comprises a two-dimensional array of modulator sites that operate upon incident light in order to form a two-dimensional image. LCD devices use light polarization characteristics in order to modulate each light pixel in the array. DMD devices use an array of tiny micro-mirrors to modulate individual light pixels. Each pixel in a spatial light modulator array is capable of exhibiting a variable light intensity in response to a corresponding variable analog voltage level.
In operation, analog image data is provided to the spatial light modulator array in a sequential scan, with analog voltages provided for a block of successive pixels at one time. For example, a typical LCD device is designed to accept a 16-pixel block of analog voltages at a time, as corresponding drive voltages for 16 pixels. Repeated delivery of analog drive voltages, 16 channels at a time, drives the LCD spatial light modulator so that a complete array containing thousands of pixels can be refreshed several times per second in order to provide successive frames of image data at a refresh rate required for motion picture imaging.
For an array containing many thousands of pixels, it can be appreciated that there will be variations in response between pixels. Without correction in some form, differences in pixel response can cause patterning, streaking, and a number of related undesirable image anomalies. Where such differences are a result of drive voltage variations, undesirable patterning image anomalies can be particularly pronounced, degrading the imaging performance of a projector apparatus.
A number of methods have been used to adjust for pixel-to-pixel variations in order to calibrate the spatial light modulator so that a more uniform response can be provided. A conventional approach for spatial light modulator calibration is to measure the light output of each individual pixel component, given a standard input signal level. An illustration of this method is disclosed, for example, in U.S. Pat. No. 6,188,427 (Anderson et al.) in which an automated calibration system is provided for an array of light-emitting elements. Correction values for zones of pixels are stored in a look-up table (LUT) for use during printing operation. Similarly, U.S. Pat. No. 6,014,202 (Chapnik et al.) discloses a spatial light modulator calibration method that measures light intensity output from a spatial light modulator and compensates by adjusting drive voltage. A number of patents disclose methods for compensating for weak or otherwise defective pixels and for correcting for fringe effects and near-neighbor pixel interaction, such as U.S. Pat. No. 4,636,039 (Turner) and U.S. Pat. No. 5,719,682 (Venkateswar).
While conventional methods are useful in profiling the pixel-by-pixel response of a spatial light modulator and in compensating for pixel-by-pixel variation, there are some drawbacks to these methods. Notably, conventional methods that measure light output attempt to correct for differences only at the spatial light modulator itself. However, there may be underlying causes that would be better corrected earlier in the imaging signal chain, not at the spatial light modulator itself. Specifically, variations in channel driver input voltages will potentially have a much more pronounced effect on output image quality that will variations in pixel-to-pixel response. For example, in an imaging system where an LCD has a 16-channel input driver, one or more of these input channels may be weak. This would cause every 16th pixel to be driven at a lower voltage level, resulting in objectionable streaking or patterning in the output image.
Channel-to-channel differences can also develop over time, as components age. Thus, conventional manual methods for channel equalization, using potentiometer adjustment, have limitations with respect to cost and practicality. An alternate approach, using only high-precision electronic components can be costly and may not adequately solve the problem of providing equalized channel driver voltages. Therefore, it can be seen that there is a need for an automated method for driver channel equalization in a multichannel imaging apparatus using a spatial light modulator.
It is an object of the present invention to provide a multichannel driver circuit for controlling each of a plurality of channels of a spatial light modulator, the circuit comprising:
(a) a control logic processor for providing as output for each channel, a digital pixel value based on input image data and digital calibration data;
(b) a reference voltage and correction generator that provides as output for all channels a positive half-cycle reference voltage and a negative half-cycle reference voltage, and that provides as output for each channel, based on said digital calibration data:
(b1) a gain compensation value;
(b2) a positive half-cycle correction voltage; and,
(b3) a negative half-cycle correction voltage;
(c) for each channel, a channel signal generator for accepting as input said digital pixel value and said gain compensation value and for providing as output a conditioned gain analog pixel voltage;
(d) for each channel, a flipper circuit for accepting as input said conditioned gain analog pixel voltage, said positive half-cycle reference voltage, said positive half-cycle correction voltage, said negative half-cycle reference voltage, and said negative half-cycle correction voltage and for providing, as output:
(d1) a positive half-cycle pixel driver output voltage obtained by conditioning said positive half-cycle reference voltage by said positive half-cycle correction voltage and summing the result with said conditioned gain analog pixel voltage;
(d2) a negative half-cycle pixel driver output voltage obtained by conditioning said negative half-cycle reference voltage by said negative half-cycle correction voltage and summing the result with the additive inverse of said conditioned gain analog pixel voltage;
(e) a comparator for performing the following operations for each channel:
(e1) sampling said positive half-cycle pixel driver output voltage from said flipper circuit and providing a first output signal to said control logic processor indicative that said positive half-cycle pixel driver output voltage is substantially equal to said positive half-cycle reference voltage;
(e2) sampling said negative half-cycle pixel driver output voltage from said flipper circuit and providing a second output signal to said control logic processor indicative that said negative half-cycle pixel driver output voltage is substantially equal to said negative half-cycle reference voltage.
According to another aspect, the present invention provides an imaging system that uses a spatial light modulator having a plurality of signal channels, wherein an apparatus for obtaining a channel correction signal for calibrating each channel comprises:
(a) for all channels, a standard signal generator for providing a standard reference video black-level signal;
(b) a channel correction signal generator for generating, for each of said plurality of signal channels, a channel correction signal corresponding to a digital input value;
(c) a comparator for comparing a summed signal comprising said channel correction signal and a channel video black-level signal against said standard reference video black-level signal, and for providing a comparator output signal indicative that said summed signal is equal to said standard reference video black-level signal;
(d) a multiplexer for selectively switching said summed signal to said comparator, based on a channel selector signal;
(e) a control logic processor for providing said channel selector signal to said multiplexer, for accepting said comparator output signal, and for executing a control program that obtains said channel correction signal for each channel and stores said channel correction signal in a memory.
Another embodiment of the present invention provides, in an image display apparatus employing a plurality of channel drivers for a spatial light modulator having a plurality of channels, a method for calibration of each individual channel driver, the method comprising:
(a) over the positive half-cycle of a drive signal for said each individual channel, obtaining a positive channel correction signal by iteratively comparing a positive summed channel driver signal, said positive summed channel driver signal comprising a positive black-video channel driver signal added to a positive channel correction signal, against a positive standard signal and incrementing said positive channel correction signal until said positive summed channel driver signal equals said positive standard signal, at which time said positive channel correction signal is stored;
(b) over the negative half-cycle of a drive signal for said each individual channel, obtaining a negative channel correction signal by iteratively comparing a negative summed channel driver signal, said negative summed channel driver signal comprising a negative black-video channel driver signal added to a negative channel correction signal, against a negative standard signal and incrementing said negative channel correction signal until said negative summed channel driver signal equals said negative standard signal, at which time said negative channel correction signal is stored;
(c) obtaining a gain level by iteratively comparing a channel white-level signal against a standard white-level signal and incrementing a gain signal, until said channel white-level signal equals said standard white-level signal, at which time said gain signal is stored;
wherein said positive channel correction signal, said negative channel correction signal, and said gain signal serve to calibrate said each individual channel driver.
A feature of the present invention is an automated sequence for multichannel calibration available upon command. This sequence can be automatically initiated at equipment power-up or used whenever necessary to maintain equipment performance over time and compensate for possible component drift.
It is an advantage of the present invention that it provides a method for equalizing driver signal levels that is inherently adaptable to modular component design. The present invention allows replacement of a spatial light modulator component, for example, where the only additional calibration needed would be for the spatial light modulator component itself.
It is an advantage of the present invention that it provides circuitry and logic commands that allow automated channel driver calibration without need of additional instrumentation.
It is a further advantage of the present invention that it provides method for device calibration that is direct, and is less expensive than conventional methods that measure light output.
These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.